Question

CMR:

a,\(x^2+xy+y^2+1>0\) với mọi x

b,\(x^2+4y^2+z^2-2x-6z+8y+15\) với mọi x

Answer 1

a) \(\left(x\right)^2+2\left(x\right)\left(\frac{1}{2}\right)+\left(\frac{1}{2}\right)^2-\left(\frac{1}{2}\right)^2+1\)

\(=\left(x+\frac{1}{2}\right)^2+\frac{3}{4}\)

Vì \(\left(x+\frac{1}{2}\right)^2\ge0\forall x\)

Nên \(\left(x+\frac{1}{2}\right)^2+\frac{3}{4}>0\forall x\)

b) \(\left(x^2-2x+1\right)+\left(4y^2+8y+1\right)+\left(z^2-6z+9\right)+4\)

\(=\left(x-1\right)^2+\left(2y+1\right)^2+\left(z-3\right)^2+4\)

Vì \(\left(x-1\right)^2+\left(2y+1\right)^2+\left(z-3\right)^2\ge0\forall x,y,z\)

Nên \(\left(x-1\right)^2+\left(2y+1\right)^2+\left(z-3\right)^2+4>0\forall x,y,z\)